Variations of splittings of vibronic Jahn-Teller states in asymmetrically deuterated benzenes

Measurements of the Rydberg absorption spectrum converging to the first ionization threshold of asymmetrically deuterated benzenes are reported. Evidence of splittings of the 0→0 bands in the R′ series upon reduction of the nuclear symmetry to D_2h or C_2v by deuteration is revealed. No such effect is present in the R series. The 0 → 1 bands of v₁₈(e_2g) in the R′ series exhibit a much smaller splitting. A theoretical analysis predicting all of the facets of these phenomena is presented. These observations enable an independent assignment of the R and R′ series previously obtained on the basis of splitting in the hot band structure of the R′ series due to quadratic Jahn-Teller activity of the out-of-plane mode v₂₀(e_2u) which cannot be linearly J-T active. These results conform with the direct observations of splittings of Jahn-Teller degeneracy obtained in benzene Rydberg series converging to the second ionization threshold upon asymmetric deuteration.